1. Field of the Invention
The invention relates to an aluminum-nitride sintered body mainly used in semiconductor substrates, particularly an aluminum-nitride sintered body that has both high thermal conductivity and considerably improved mechanical strength, a fabricating method for the same, and a semiconductor substrate comprising the same.
2. Description of the Background Art
Having high thermal conductivity, superior electrical insulating properties, and a coefficient of thermal expansion close to that of silicon, aluminum-nitride sintered bodies have been used for heat-dissipating substrates for semiconductors. This has expedited studies on the enhancement of thermal conductivity of aluminum nitride aiming at the improvement of heat dissipation, which has enabled the production of sintered bodies having a thermal conductivity of more than 200 W/mK in recent years.
However, such highly heat-conductive aluminum-nitride sintered bodies are composed of sintered grains having an extremely large grain size of 7 to 8 .mu.m or more, so that their mechanical strength is undesirably low. Means to suppress this reduction in mechanical strength have been studied and disclosed in unexamined published Japanese patent applications Tokukaihei 6-206772, Tokukaihei 6-329474, and Tokukaihei 7-172921.
Tokukaihei 6-206772, for instance, has disclosed a method for fabricating an aluminum-nitride sintered body by mixing an AlN powder having a primary particle size of 0.01 to 0.3 .mu.m and impurity oxygen of more than 1.5 wt. % with a 7 wt. % or less sintering agent and sintering the resultant mixture at 1600.degree. C. or below in a non-oxidizing atmosphere. According to the patent application, a compound of alkaline-earth metal and/or rare-earth metal, a small number of aluminum compounds, and a small number of silicon compounds are used as the sintering agent; a compound containing transition metal elements may be used as a coloring agent, as required.
The patent application also has disclosed another method wherein an AlN powder having a primary particle size different from the foregoing range, for example 0.5 to 1.0 .mu.m, is used concurrently. As an example of the embodiment it has presented the production of an aluminum-nitride sintered body with an average grain diameter of 1.5 .mu.m having a thermal conductivity of 220 W/mK and a 4-point bending strength of 45 kg/mm.sup.2 (459 MPa).
Tokukaihei 7-172921 has disclosed a method for fabricating an aluminum-nitride sintered body by mixing a fine AlN powder having an oxygen content of 1.5 wt. % or less and an average particle diameter of 0.5 to 2 .mu.m with at least one kind of compound of 3A- or 2A-group elements in the periodic table, a small number of Si constituents, a small amount of Al.sub.2 O.sub.3, and oxides of transition metal elements as required and sintering the resultant mixture at 1650 to 1900.degree. C. in a non-oxidizing atmosphere. According to the patent application, the AlN sintered body has a thermal conductivity of 150 W/mK or more, a 3-point bending strength of 490 MPa or more, and a fracture toughness of 2.8 MN/m.sup.3/2 or more.
As mentioned above, various means have been offered to suppress the reduction in mechanical strength of a highly heat-conductive aluminum-nitride sintered body. However, no means can be said to be suitable for mass production because each individual means requires strict control of a number of manufacturing conditions such as an aluminum-nitride material powder's oxygen content and particle diameter, a sintering agent, and other additives.
As for aluminum-nitride material powders in particular, the oxygen content and the particle diameter differ with manufacturing methods and manufacturers. However, the patent applications cited above do not specify the material powders. Moreover, only a few types of low-cost aluminum-nitride powders obtained by the conventional solid-phase method satisfy the manufacturing conditions of the material powder required in each of the foregoing patent applications.
As for aluminium-nitride, it is known that the properties of the sintered bodies obtained from the material powders having the same average particle diameter are greatly different depending on the lattice defects of the material powders. For example, conventional powders obtained by the solid-phase method are prone to contain lattice strains and other defects in the powder particles due to volume expansion and contraction at the stage of powder synthesis. Because the defects remain in the crystal grains in the sintered body, the defects are liable to act as the starting point of fracture when external stress is applied, reducing the mechanical strength of the sintered body.